The major objective of this project is to uncover the enzymatic steps involved in various genetic rearrangement reactions and to study the mechanism of action of the enzymes involved. The mechanism of the transposition-replication reaction of bacteriophage Mu is studied under this project as a model system. A critical step in the Mu transposition reaction is a pair of DNA strand transfers which generate a branched DNA intermediate. Efficient formation of this intermediate requires Mu A, Mu B and E. coli HU proteins along with ATP and Mg++. The Mu A protein binds to the Mu end DNA sequences on the donor DNA by one domain and to the internal Mu operator DNA site by another domain to form a special protein-DNA complex necessary to initiate the Mu DNA strand transfer reaction. Next, a pair of single strand cuts are made to expose the 3' ends of the Mu sequence to yield cleaved donor DNA with tightly associated Mu A proteins. This protein-DNA complex efficiently captures a second DNA molecule provided it is bound by Mu B protein. A staggered cut is introduced into the target DNA and each 5' end is joined to each of the 3' ends of the Mu end sequences by a concerted DNA cutting and joining reaction. Evidence has been obtained that this step of the reaction takes place by one step transesterification mechanism. The Mu B protein, an ATPase, selectively stimulates the utilization of intermolecular target DNA molecules which do not carry Mu end sequences. The Mu B protein dissociates selectively from the DNA molecule to which Mu A protein is bound in a process that depends on hydrolysis of ATP. Kinetic aspects of this energy transduction system are studied.